Microcapsules and microencapsulation technology are old and well known and their commercial applications varied. Microcapsules have played a significant role in various print technologies where a paper or other like substrate is coated with microcapsules containing ink or an ink-forming or inducing ingredient which microcapsules release the ingredient, generating an image, when fractured by pressure, as by a printing press or a stylus. Microcapsules have also played a significant role in various adhesive and sealant technologies including the encapsulation of solvents for solvent swellable/tackified preapplied adhesives whereby fracture of the microcapsules releases the solvent which softens or tackifies the adhesive to enable bonding and which re-hardened upon evaporation of the solvent. In other adhesive and sealant applications, the microcapsules contain one or more components of a curable or polymerizable adhesive or sealant composition which, upon release, leads to the cure or polymerization of the adhesive or sealant. In all of these early applications, functionality and efficacy, especially for long term storage and utility, is dependent upon the integrity of the microcapsule walls where the sought after integrity pertains to both strength, so as to avoid premature fracture, as well as impermeability, so as to prevent leakage and/or passage of the contents of the microcapsule through the microcapsule walls while also have easily attained break points to allow them to perform their function when intended by, e.g., simply screwing parts coated with coatings containing the microcapsules together.
Evolution of microencapsulation technology has led to many new commercial applications for encapsulated material, including applications that require microcapsules that fracture more readily, with less pressure, but not prematurely. Other applications require microcapsules that specifically allow for a controlled, slow release or permeation of the contents from within the microcapsules without the need to actually fracture the same. For example, perfume containing microcapsules are oftentimes applied to advertising inserts in magazines so that the reader can sample the smell of the perfume. Here strength is needed to avoid premature fracturing of the microcapsules due to the weight and handling of the magazine; yet, the microcapsules need ease of fracture so that the reader can simply scratch the treated area to release the contents of the microcapsule. At the same time, it is desirable to allow for some release of the contents, even without fracturing, to induce the reader to want to scratch the sample to get a more accurate sense of the smell.
Microcapsules are also finding increasing utility in laundering and fabric treatments: an application that requires both strength and defined release or break points. For example, a number of products exist wherein microcapsules of various ingredients, including perfumes, are applied to strips of a fabric material and added to the dryer wherein the tumbling action and/or heat of the dryer causes the microcapsules to fracture and/or become more permeable, releasing the ingredients which, in a volatilized state, permeate and deposit upon the contents of the dryer. This methodology applies that “fresh out of the dryer” smell, but is short lived as the perfume continues to volatilize from the treated fabric. Other products exist whereby microcapsules containing perfumes, odor controlling or masking materials, and other ingredients are applied directly or indirectly to the fabric, especially apparel, to provide a longer lived freshness to the same. Here, the integrity of the microcapsules is such that the microcapsules will not readily break during washing and handling, but will break during normal use and wearing of the garment which allows for the continued release of the contents of the microcapsules.
Despite the historical need for a break or fracturing of microcapsules, new potential applications are developing where capsule strength, specifically high strength, is becoming more and more critical. In these applications and intended applications, the microcapsules must be able to withstand conditions of high pressure, including pressures up to 100 psi, even 200 psi, or more as well as high temperatures, e.g., in excess of 150° C., even 200° C., without fracture and without an increase in the permeability of the shell wall, at least not until specific trigger events are attained which allow for such fracture or increased permeability. Still other applications require microcapsules that are not intended to break or release their contents at all. These demands are especially necessary for microcapsules containing phase change materials. For example, microcapsules containing phase change materials are being incorporated into fabrics so as to allow the fabric to draw heat away from the body. Most often these fabrics are incorporated into sporting garments and must withstand harsh wear due to physical exertion by the wearer, repeated washings with strong cleaning materials, etc., all without fracturing the microcapsules containing the phase change material. Other uses for microcapsules containing phase change materials include medical devices, construction materials, bedding, refrigerated transport, energy storage, cooling fluids, absorptive chillers such as for circuitry, solar devices, and applications where temperature moderation is desired. Where the microcapsule core is phase change material, uses can include such encapsulated materials in mattresses, pillows, bedding, textiles, sporting equipment, medical devices, building products, construction products. HVAC, renewable energy, clothing, athletic surfaces, electronics, automotive, aviation, shoes, beauty care, laundry, and solar energy.
Despite all the advances and improvements, there is still a need for improved specialty microcapsules that provide a suitable mix of containment or release/permeability characteristics and physical properties, especially high strength properties, for today's demanding applications. This is especially so in the area of perfumes and other odiferous ingredients, particularly in relation to fabric, textile and garment treatment, where controlled release and longevity as well as capsule strength and integrity are necessary.
Various methods and shell wall forming compositions have been proposed in the art in an effort to make microcapsules capable of withstanding certain harsher conditions, particularly higher temperatures. For instance, Sinclair (U.S. Pat. No. 4,396,670) encapsulated hydrophobic liquids using aminoplast resin capsules prepared from melamine formaldehyde pre-condensate; however, these become more permeable at elevated temperatures and may release formaldehyde. Jahns et. al. (U.S. Pat. No. 6,200,681) encapsulated latent heat storage materials in a shell formed by free radical polymerization of a monomer mixture comprising 30-100% of one or more C1-C24 alkyl esters of (meth)acrylic acid, 0-80% of a water insoluble or low solubility bi- or polyfunctional monomer and 0-40% of other monomers; however, the specific microcapsules described do not possess sufficient strength and will enable too much loss of core material at higher temperature. Weston et. al. (U.S. Pat. No. 6,716,526) prepare microcapsules having a shell comprising a copolymer formed from a monomer blend of 30-90% methacrylic acid, 10-70% of an alkyl ester of (meth)acrylic acid whose homopolymer has a Tg in excess of 60° C. and 0-40% other ethylenically unsaturated monomer. Although an improvement in that the permeability is improved even at high temperatures, these too lack the desired strength characteristics. Building on Weston et. al., Grey (U.S. Pat. No. 8,784,984) microencapsulates hydrophobic core materials in a polymer shell comprising the reaction product of a monomer mixture containing 1-95% of at least one hydrophobic mono-functional ethylenically unsaturated monomer, 5-99% of at least one polyfunctional ethylenically unsaturated monomer, and 0-60% of other mono-functional monomers wherein a hydrophobic polymer is incorporated into the monomer mixture prior to the polymerization thereof.
Despite the advances, the need still exists for improved microcapsules for hydrophobic materials and, perhaps more importantly, a more convenient and simpler method for their production. Most especially there is still a need for high integrity microcapsules that will withstand the forces and environments associated with their intended use, particularly in the encapsulation of phase change materials, without rupture or compromising the integrity of the microcapsule walls.